1. Field of the Invention
The present invention relates to spin injection sources with high spin injection efficiencies and their manufacturing methods.
2. Description of the Background Art
Spintronics is an emerging technology that deals with the intrinsic spin of the electron and aims for creation of new high-functional devices utilizing the spin, and has potential to break the limits of conventional semiconductor devices. For example, the application of spin accumulation devices (devices utilizing the spin accumulation effect) for reading heads on hard disk drives or nonvolatile magnetic memories is expected to realize memory density of 1 Tbit/inch2. However, the output signal of a conventional spin accumulation device is reported to have a value of several μV typically, and of several tens of μV at best. Increasing the weak signal is a key factor for practical application.
In a lateral spin valve, the spin is injected and accumulated in the nonmagnet by passing an electric current through the junction interface between the nonmagnet and ferromagnet. There are two types of junction interfaces in the lateral spin valve: ohmic junction and tunnel junction. The ohmic junction has a ferromagnet and a nonmagnet directly joined together, and its interface resistance is small. Here, since the ferromagnet has a small spin resistance and the nonmagnet has a large spin resistance, there is a spin resistance mismatch. Due to the mismatch, an effective spin injection is difficult, and the spin accumulation resistance change ΔRS is as small as 1 mΩ. The tunnel junction has an insulating layer between the ferromagnet and the nonmagnet, and its interface resistance is large. Since the tunnel junction overcomes the spin resistance mismatch, it may exhibit larger ΔRS. However, since an increase in the applied voltage decreases the spin injection efficiency, the spin valve signal voltage may not become so large.
In Y. Fukuma, et al., “Enhanced spin accumulation obtained by inserting low-resistance MgO interface in metallic lateral spin valves”, Applied Physics Letters, vol. 97, 012597 (2010), the inventors have reported that even if a layer of lower resistance MgO than that in the conventional tunnel junction is used in a spin injection device made of a ferromagnet (NiFe) MgO, a nonmagnet (Ag), the problem of the spin resistance mismatch may be overcome. The electric resistance of the MgO layer is two orders smaller than that used in the conventional tunnel junction. Since the spin accumulation voltage is a product of the spin accumulation resistance change and the current (ΔV=ΔRS×I) and a larger current can be applied to low-resistance MgO, the spin valve signal may become larger.